'Dome' is a temporary method of containing gulf oil spill

The 78-ton steel containment dome that crews lowered over the Deepwater Horizon site on Thursday night represents the best immediate chance to slow the oil spilling into the Gulf of Mexico from the blown-out well.

But even if it works — a big "if" that may not play out for days — it's still a temporary measure subject to weather and other conditions.

"A dome might slow the leak, but it can't stop it," said Dr. Philip Johnson, a professor of petroleum engineering at the University of Alabama.

The only permanent solution is to drill relief wells to shut off the flow, Johnson and other experts say. And BP says that will take three months. Because of that, a half-dozen other methods -- from burning the oil to dispersing it with chemicals, continue at full speed.

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Workers lowered the four-story dome onto the seabed surface late Thursday night, but said it will be Sunday or Monday before they will know if it's working.

Oil has been leaking in three places since the April 20 explosion. One small leak was capped Wednesday. The containment box will be lowered over a much bigger leak in a pipe that's responsible for about 85 percent of the oil that's coming out.

"This kind of system worked very effectively after Hurricane Katrina," said Greg McCormack, director of the Petroleum Extension Service at the University of Texas. "But it was in much shallower waters, mostly less than 200 feet deep."

At 5,000 feet it will be much harder.

"It's pitch black down there. There are no divers. And there are all kinds of currents," McCormack said.

If the box being lowered Thursday can contain the bigger leak, a second box being built may be used to stop the smaller leak at the blowout preventer.

Even with two domes in place, the method depends on piping the oil up to a ship, which will siphon it into smaller ships to be carried away. But, Johnson notes, "if a hurricane comes, you're in trouble." Hurricane season starts in June.

Relief wells are the best solution, the experts say.

"It's the standard method when you've lost control of high-pressure wells," said Greg Pollock, head of the oil spill division of the Texas General Land Office.

BP began drilling the first of two planned relief wells near the broken well on Sunday. Tony Hayward, BP's group chief executive, estimates it will take three months to complete.

One other alternative BP engineers are considering is to try to plug the leaking well from the top instead of drilling a relief well to cap it from the bottom. That would take two to three weeks.

Three months to drill a relief well is "an optimistic estimate," says Dr. Don Van Nieuwenhuise, geology professor at University of Houston who helped drill two relief wells for an earlier Gulf oil well blowout. The oil in the area beneath the BP well is trapped in shale under great pressure.

Drilling into it could create new leaks if not done carefully, he said.

Ever since the oil rig exploded, dozens of BP and Coast Guard ships have been cruising through the oil slick on the surface of the Gulf spraying dispersants into it. Dispersants are mixtures of solvents, surfactants and other compounds that break up the surface tension of the slick, making the oil more soluble in water.

Wave action pulls the oil apart into even smaller droplets, which remain suspended beneath the water or fall to the ocean floor. It helps protect onshore birds and animals, but wildlife experts fear its effects on fish and other animals living beneath the sea, according to the National Academics of Science.

In another novel attempt to reduce oil damage, BP workers on Wednesday injected about 3,000 gallons of dispersant directly into the leaking well on the seabed.

So far, Coast Guard and BP vessels have used 190,285 gallons of dispersant and have another 55,611 gallons available, according to the Deepwater Horizon Response Operation.

The use of dispersants has won only grudging approval from environmentalists and even petroleum engineers.

"Dispersants are chemicals. Chemicals aren't good in the environment. It's a trade-off," McCormack said.

Meanwhile, BP, the U.S. Coast Guard and an army of volunteers are using several other strategies to stop damage from the gushing oil.

Controlled burning: On Thursday, favorable weather conditions finally allowed cleanup crews to conduct a controlled burn of oil on the surface. An earlier successful burn took place April 28, destroying thousands of gallons of oil, but rough weather had frustrated several attempts since.

In a controlled burn, boats maneuver through the oil slick towing buoyant, fire-resistant booms to gather the oil into a thick, flammable pool. When a "boomful" of oil is gathered, it is towed away and ignited. When an oil slick burns, residue hardens and drops to the ocean floor. Oil-skimming boats: BP and the Coast Guard have at least 35 ships in the Gulf skimming the oil from the surface and pumping it into barges.

"Rough seas can limit its effectiveness, but you have to use every method available," Pollock said.

Floating booms: These are miles-long, 20-inch-tall devices of vinyl fabric with a foam float stitched inside for buoyancy that can be stretched along the water. They can help contain oil slicks at sea, redirect them into planned areas for recovery or disposal and hold them back from environmentally sensitive areas.

The Deepwater Horizon Response Operation reports that 535,870 feet of booms had been deployed, with another 664,9891 feet available. They are being used offshore in the Gulf to redirect the oil slick, and near shore to protect shorelines at six locations including Pensacola. For days, rough seas have disrupted many of the booms, hurting their effectiveness.